Case of the Month: Scuticociliatosis in seahorses


Kerstin Erles

Board Certified Anatomic Pathologist

Multiple sudden deaths were reported in a population of captive seahorses (Hippocampus abdominalis).

Histological examination revealed numerous ovoid to pear-shaped ciliated protozoal organisms measuring approximately 25×15 µm with a single nucleus. These were most numerous in subcutaneous fibrous connective tissue and muscle, but were also found within other tissues including the brain. The histological features were consistent with scuticociliates.
















Protozoa are present in the periocular connective tissue and brain.

Background: Scuticociliates are free-living protozoa, causing disease in a wide range of marine fish species (1, 2). Often they act as opportunistic pathogens, but they have also been associated with mass mortality events, most recently in sea urchins (3). Environmental changes such as temperature fluctuations or poor water quality may predispose to infection.

Infected individuals may show lethargy, as well as areas of skin depigmentation or ulceration. Initially the organisms may be limited to the skin and gills but may then progress to systemic infections (4).

The infection may be diagnosed using wet mounts of skin or gills, but these may not yield sufficient protozoa, and often histopathology is required for diagnosis.

  1. Di Cicco E,  Paradis E,  Stephen C,  Turba ME, and Rossi G. Scuticociliatid ciliate outbreak in Australian pot-bellied seahorse Hippocampus abdominalis (Lesson , 1827): Clinical signs, histopathologic findings and treatment with metronidazole Journal of Zoo and Wildlife Medicine44(2), 435-440, (1 June 2013).
  2. Stidworthy MF, Garner MM, Bradway DS, et al. Systemic Scuticociliatosis (Philasterides dicentrarchi) in Sharks. Veterinary Pathology. 2014;51(3):628-632. doi:1177/0300985813492800
  3. Hewson I et al. A scuticociliate causes mass mortality of Diadema antillarum in the Caribbean Sea. Sci Adv. 2023 Apr 21;9(16):eadg3200. doi: 10.1126/sciadv.adg3200
  4. Edward J Noga. Fish Disease: Diagnosis and Treatment. 2nd Wiley-Blackwell 2010. Chapter 8, pg. 141-143.